Steel strip is normally produced at a steel mill in coil form, and then transported to a remote location at which the strip will be cut, formed, and frequently painted or otherwise coated. The steel strip may be oiled, or otherwise protected from oxidation, such as by having phosphate or chromate solution applied thereto. Additionally, the steel strip may be a galvanized strip, such as produced by hot dipping or through an electrical reaction known as electrogalvanizing. Yet another zinc-containing coating is Galvalume.RTM., a composition containing zinc and aluminum. Once received at the manufacturing facility, then the chromate, oil, phosphate or other protective coat is normally removed and the strip further processed.
The application of a chromate solution to steel strip not only is relatively expensive, but also create costs related to the need to dispose of the material in an environmentally acceptable manner. The environmental issues arise not only at the steel plant where the chromate is applied, but also at the manufacturing facility where the chromate is removed. Moreover, there also are costs related to application and removal of the chromate material.
In addition to environmental issues raised by the treatment of the steel, end users continuously seek improved raw materials to enhance the marketability of the resulting end product. Carbon steel strip frequently is used to manufacture steel siding and steel roofing, with those produces necessarily ultimately being located in environments where corrosion and oxidation may occur. Protective coats, such as paint, are usually applied to the finished product to prevent corrosion and oxidation (rust). The paint may not provide an adequate barrier to moisture, particularly if the surface should become scratched or otherwise broken. In that event, moisture in the air will react with the steel and form rust, which either is unsightly or may ultimately cause a product failure.
Previous efforts to apply a protective coating to steel at the steel mill have not been entirely successful, because of environmental issues, end user or manufacturer requirements, or simply cost. For example, not only is it desirable that the protective coating be relatively inexpensive to apply, but same should provide a durable surface which may be painted or otherwise protected. Thus, good adhesion to paint is required. Additionally, as noted, the strip frequently is formed by the manufacturer, so that good adhesion of the protective coat to the strip is also desirable, so that the protective coat will remain after the forming operation.
Plasma polymerization of HMDSO under the influence of non-reactive gases has been used in the past to apply high carbon siloxane-like films to electrogalvanized steel. The resulting films were thereafter post-treated with an oxygen etch, so that the surface would repel water and have improved corrosion resistance.
Similar efforts have been used to permit structural bonding of aluminum through epoxy adhesives. The adhesion between the epoxy adhesives and the siloxane films was poor, and post-treatment with an oxygen plasma revealed that the films were inherently weak. Formation of a silica-like film under the influence of an oxygen carrier provided fair initial strength to the smooth aluminum surface, although the strength was still less than smooth aluminum with no primer. The films even then had poor durability, which could be improved by acid etching the aluminum to a roughened state prior to application of the plasma polymerized film.
Those skilled in the art will recognize that there is a need for a protective coat which may be applied to zinc clad steel strip at the steel mill to overcome the problems of the prior art, and to enhance the usability of the resulting steel product. The disclosed invention meets these and other needs in the art by causing a silica-like coating to be adhered to the zinc coated steel strip, with the silica protective coat being deposited through plasma polymerization of HMDSO under the influence of high power and a large excess of oxygen.